Genetics

Genetics is the scientific study of heredity.Chapter 11Sections 1-3

The Work of Mendel 1822 – Gregor Mendel used

purebred pea plants to study the inheritance of traits.

Fertilization – the process in sexual reproduction in which male & female reproductive cells join to form a new cell.

Mendel’s Experiment with Pea Plants Mendel used the pea flower because they

have both male & female parts. Self-pollinating – sperm cell in pollen

fertilize the egg cells in the same flower. The offspring inherit characteristics from a single parent. Ex. Pea Plant flowers

True-breeding- the peas plant flowers were self-pollinating and could produce offspring identical to themselves.

Experiment I: Pea Plant Mendel had to prevent self-pollination so

he made a controlled cross between plants. He cut away the male parts & dusted pollen from another plant on the flower.

Cross-pollination – seeds produced are the offspring of 2 different parents.

Mendel studied a few traits, or specific characteristics of the pea plant such as color & height.

Mendel Studies 7 Traits / Characters pg. 264

Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height

Genes and Dominance Mendel crossed 2 plants with different

characters, or forms, for the same trait. Ex. 1 tall & 1 short The plants that grew were hybrid. Hybrid – are the offspring of crosses

between parents with different traits. Parent plant – P generation First set of offspring – F1 generation

Experiment I Concluded: Traits are inherited through the passing

of factors from parents to offspring. The hybrid plants looked like only 1

parent and the character of the other parent seemed to disappear.

Each trait is controlled by 1 gene.Alleles – controls the different forms of a

gene.Genes – chemical factors that determine

traits.

Principle of Dominance States that some alleles are

dominant & others are recessive. Whenever a living thing inherits a

dominant allele, that trait is visible. The effects of a recessive allele are

not seen if the dominant allele is present.

Experiment II: Segregation Mendel crossed a tall plant

(dominant) with a short plant (recessive), the F1 plant inherited an allele for tallness from the tall parent & an allele for shortness from the short parent. Pg. 265

What happened to the recessive allele? Mendel allowed his hybrid plants to

self-pollinate. Some showed recessive traits, the

recessive traits did not disappear. Earlier, the dominant masked the

recessive, so it was not visible.

Experiment II Conclusion Alleles for the same trait can be

separated. Segregation – When sex cells, or

gametes, are formed. Each gamete carries only 1 copy of each gene.

Therefore, each F1 plant produces 2 types of gametes (some with an allele for tallness & some with an allele for shortness). Ex. T, t, T, t = TT, Tt, Tt, tt

Probability & Punnett Squares Probability – the likelihood that a

particular event will occur. Ex. Flipping a coin. The probability

that it will land on tails is ½.

The Principles of Probability It is used to predict the traits of the

offspring produced by genetic crosses.

Mendel used a Punnett Square to show the gene combinations that might result from a genetic cross.

Punnett Squares: The F1 parent is shown along the top

& left sides. The possible F2 offspring are in the 4

boxes that make up the square. Letters represent alleles. T is dominant allele for tallness. T is recessive allele for shortness. Pg. 268

The Punnett Square show: All the possible results of a genetic

cross. The genotypes of the offspring. The alleles in the gametes of each

parent.

Punnett Squares Homozygous – organisms that have 2

identical alleles for a particular trait; they are true-breeding. Ex. TT or tt

Heterozygous – organisms that have 2 different alleles for the same trait; they are hybrid. Pg. 269

Genotype – genetic make up. Phenotype – Physical characteristics;

don’t have the same genotype.

Probability of SegregationPg. 268-269

¼ (25%) of F2 plants have 2 alleles for tallness (TT).

½ (50%) of F2 plants have 1 allele for tallness & 1 allele for shortness (Tt).

¾ (75%) of F2 plants are tall because the allele for tallness is dominant over allele for shortness.

Overall, there are 3 tall for every 1 short plant in F2 generation; Ratio of 3:1

Independent Assortment states that during gamete formation,

genes for different traits separate with out influencing each other’s inheritance.

Helps account for genetic variations. Ex. Pea shape & pea color.

Experiment III The Two-Factor Cross: F1

Mendel wanted to see if genes that determine 1 trait have anything to do with genes that determine another.

He followed 2 different genes as they passed from one generation to the next.

Mendel crossed true-breeding plants - round yellow peas (RRYY) with wrinkled green peas (rryy).

The F1 offspring were all round & yellow showing that both were dominant alleles. The genotype is RrYy. Pg. 270

Experiment IIIThe Two-Factor Cross: F2

The F1 plants were all heterozygous with a genotype of RrYy.

The F1 plants were segregated to each other to produce an F2

generation. Mendel compared the variations in

the seeds.

Experiment III Conclusion Mendel found some offspring that

were:• Round / yellow (P phenotype)• Wrinkled / green (P phenotype)• Many with combinations of alleles (not

found in either parent)• This showed that the alleles for seed shape

segregated independently of those for seed color (independent assortment). Pg. 271

Summary of Mendel’s Principles Genes are passed from parents to their

offspring. Some forms of the gene may be dominant

and others may be recessive. Each adult has 2 copies of each gene –

one from each parent. The genes are segregated from each other when gametes are formed.

Alleles for different genes usually segregate independently of one another.

Different Patterns of Dominant & Recessive

Incomplete Dominance – 1 allele is not completely dominant over another. Ex. Red flower (RR) & white flower (WW), F1

is a pink flower (RW). Pg. 272

Codominance – both alleles contribute to the phenotype.

Ex. A cross of a black chicken (BB) with a white chicken (WW) will produce all speckled offspring (BBWW); colors appear separately.

Different Patterns of Dominant & Recessive

Multiple Alleles – have more than 2 alleles. Ex. Coat color in rabbits.

Polygenic Traits – traits controlled by 2 or more genes; “having many genes”. Ex. Variation in human skin color.

Thomas Hunt Morgan Geneticist who looked for a model

organism to use in studying genetics He chose the fruit fly because it

produces large numbers of offspring.

Genes & the Environment Mendel’s principles apply to all organisms not

just plants. Characteristics of any organism are not

determined solely by genes it inherits. Characteristics are determined by interaction

between genes and the environment.Ex. Genes affect the height and color of a sunflower but the same

characteristics are influenced by climate, soil, conditions, and availability of water.

Genes provide a plan for development, but how the plan unfolds depends on the environment.